Distributed denial of service (DDoS)

Many compromised machines, one target, a flood. Availability dies; the data may still be fine. That is not comfort during checkout.

Denial of service is drowning a service so legitimate users cannot reach it — bandwidth, CPU, or connections, take your pick. DDoS is that idea with a crowd: a botnet or hired flood from many places at once.

Absorb it upstream (CDN, provider scrubbing), keep origin addresses off the public napkin, and have an on-call who can declare 'we are being flooded' without a three-hour CAB. It is an availability incident. Do not 'fix' it by exposing admin interfaces 'just for debugging'.